This invention relates to propylene impact copolymer compositions. In particular, these unique and improved compositions can be produced using metallocene catalysts in commercial-scale processes.
Propylene impact copolymers are commonly used in a variety of applications where strength and impact resistance are desired such as molded and extruded automobile parts, household appliances, luggage and furniture. Propylene homopolymers are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
A typical propylene impact copolymer contains two phases or components, a homopolymer component and a copolymer component. These two components are usually produced in a sequential polymerization process wherein the homopolymer produced in a first reactor is transferred to a second reactor where copolymer is produced and incorporated within the matrix of the homopolymer component. The copolymer component has rubbery characteristics and provides the desired impact resistance, whereas the homopolymer component provides overall stiffness.
Many process variables influence the resulting impact copolymer and these have been extensively studied and manipulated to obtain various de effects. For example U.S. Pat. No. 5,166,268 describes a xe2x80x9ccold formingxe2x80x9d process for producing propylene impact copolymers where finished articles are fabricated at temperatures below the melting point of the preform material, in this case, the propylene impact copolymer. The patented process uses a propylene impact copolymer comprised of either a homopolymer or crystalline copolymer matrix (first component) and at least ten percent by weight of an xe2x80x9cinterpolymerxe2x80x9d of ethylene and a small amount of propylene (the second component). Adding comonomer to the first component lowers its stiffness. The ethylene/propylene copolymer second component enables the finished, cold-formed article to better maintain its shape.
U.S. Pat. No. 5,258,464 describes propylene impact copolymers with improved resistance to xe2x80x9cstress whitening.xe2x80x9d Stress whitening refers to the appearance of white spots at points of impact or other stress. These otherwise conventional propylene impact copolymers have first and second components characterized by a numerical ratio of the second component intrinsic viscosity to the first component intrinsic viscosity which is near unity.
In U.S. Pat. No. 5,362,782, nucleating agent is added to propylene impact copolymers having a numerical ratio of the intrinsic viscosity of the copolymer rubber phase (second component) to the intrinsic viscosity of the homopolymer phase (first component) which is near unity, and an ethylene content of the copolymer phase in the range of 38% to 60% by weight. These propylene impact copolymers are described as producing articles having good clarity as well as impact strength and resistance to stress whitening. The nucleating agents increase stiffness and impact strength.
U.S. Pat. No. 5,250,631 describes a propylene impact copolymer having a homopolypropylene first component and an ethylene/butene/propylene terpolymer second component. Again, the goal is to obtain high impact strength coupled with resistance to stress whitening.
Propylene impact copolymers are also used to produce films as described in U.S. Pat. No. 5,948,839. The impact copolymer described in this patent contains a conventional first component and 25 to 45 weight percent ethylene/propylene second component having from 55 to 65 weight percent ethylene. This impact copolymer composition has a melt flow of from 7 to 60 dg/min. Such films are used in articles such as diapers.
Recently, efforts have been made to prepare propylene impact copolymers using the newly developed metallocene catalysis technology in order to capitalize on the inherent benefits such catalysts provide. It is well known that homopolymers prepared with such xe2x80x9csingle-sitexe2x80x9d catalysts have narrow molecular weight distributions, and low extractables and a variety of other favorable properties associated therewith. Metallocene catalyzed copolymers have narrow composition distributions in addition to narrow molecular weight distribution and low extractables.
Unfortunately, known metallocenes are not able to provide copolymer components with high enough molecular weight under commercially relevant process conditions. The resulting propylene impact copolymers have poor impact strength compared to their conventionally catalyzed counterparts.
U.S. Pat. No. 5,990,242 approaches this problem by using an ethylene/butene (or higher xcex1-olefin) copolymer second component, rather than a propylene copolymer, prepared using a hafnocene type metallocene. Such hafnium metallocenes in general are known for producing relatively higher molecular weight polymers; however, their activities are much lower than the more commonly used zirconocenes. In any event, the second component molecular weights and intrinsic viscosities are lower than desired for good impact strength.
The present inventors have discovered new propylene impact copolymer compositions having the benefits of metallocene catalyzed polymers in addition to properties needed for high impact strength. Importantly, these polymers can be economically produced using commercial-scale processes.
The present invention provides reactor produced propylene impact copolymer compositions comprising:
(a) From about 40% to about 95% by weight Component A based on the total weight of the impact copolymer, Component A comprising propylene homopolymer or copolymer wherein the copolymer comprises 10% or less by weight ethylene, butene, hexene or octene comonomer;
(b) From about 5% to about 60% by weight Component B based on the total weight of the impact copolymer, Component B comprising propylene copolymer wherein the copolymer comprises from about 20% to about 70% by weight ethylene, butene, hexene and/or octene comonomer, and from about 80% to about 30% by weight propylene, wherein Component B:
(i) has a weight average molecular weight of at least 100,000;
(ii) a composition distribution of greater than 60%; and
(iii) an intrinsic viscosity of greater than 1.00 dl/g.
This invention also provides a process for producing propylene impact copolymer in a multiple stage process wherein Component A comprising propylene homopolymer or copolymer wherein the copolymer comprises 10% or less by weight ethylene, butene, hexene or octene comonomer is produced in a primary stage and Component B is produced in a subsequent stage, Component B comprising propylene copolymer wherein the copolymer comprises from about 20% to about 70% by weight ethylene, butene, hexene and/or octene comonomer, and from about 80% to about 30% by weight propylene, wherein at least one of Components A and/or B are polymerized using a metallocene selected from the group consisting of: rac-dimethylsiladiyl(2-iPr,4-phenylindenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[1-naphthyl]indenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[3,5-dimethylphenyl]indenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[ortho-methyl-phenyl]indenyl)2zirconium dichloride; and rac-diphenylsiladiyl(2-methyl-4-[1-naphthyl]indenyl)2zirconium dichloride.
The propylene impact copolymers (xe2x80x9cICPsxe2x80x9d) of this invention comprise at least two major components, Component A and Component B. Component A is preferably an isotactic propylene homopolymer, though small amounts of a comonomer may be used to obtain particular properties. Typically such copolymers of Component A contain 10% by weight or less, preferably less than 6% by weight or less, comonomer such as ethylene, butene, hexene or octene. Most preferably less than 4% by weight ethylene is used. The end result is usually a product with lower stiffness but with some gain in impact strength compared to homopolymer Component A.
As used herein Component A refers generally to the xylene insoluble portion of the ICP composition, and Component B refers generally to the xylene soluble portion. Where the xylene soluble portion clearly has both a high molecular weight component and a low molecular weight component, we have found that the low molecular weight component is attributable to amorphous, low molecular weight propylene homopolymer. Therefore, Component B in such circumstances refers only the high molecular weight portion.
Component A preferably has a narrow molecular weight distribution Mw/Mn (xe2x80x9cMWDxe2x80x9d), i.e., lower than 4.0, preferably lower than 3.5, more preferably lower than 3.0, and most preferably 2.5 or lower. These molecular weight distributions are obtained in the absence of visbreaking using peroxide or other post reactor treatment designed to reduce molecular weight. Component A preferably has a weight average molecular weight (Mw as determined by GPC) of at least 100,000, preferably at least 200,000 and a melting point (Mp) of at least 145xc2x0 C., preferably at least 150xc2x0 C., more preferably at least 152xc2x0 C., and most preferably at least 155xc2x0 C.
Another important feature of ICPs is the amount of amorphous polypropylene they contain. The ICPs of this invention are characterized as having low amorphous polypropylene, preferably less than 3% by weight, more preferably less than 2% by weight, even more preferably less than 1% by weight and most preferably there is no measurable amorphous polypropylene.
Component B is most preferably a copolymer consisting essentially of propylene and ethylene although other propylene copolymers, ethylene copolymers or terpolymers may be suitable depending on the particular product properties desired. For example, propylene/butene, hexene or octene copolymers, and ethylene/butene, hexene or octene copolymers may be used, and propylene/ethylene/hexene-1 terpolymers may be used. In a preferred embodiment though, Component B is a copolymer comprising at least 40% by weight propylene, more preferably from about 80% by weight to about 30% by weight propylene, even more preferably from about 70% by weight to about 35% by weight propylene. The comonomer content of Component B is preferably in the range of from about 20% to about 70% by weight comonomer, more preferably from about 30% to about 65% by weight comonomer, even more preferably from about 35% to about 60% by weight comonomer. Most preferably Component B consists essentially of propylene and from about 20% to about 70% ethylene, more preferably from about 30% to about 65% ethylene, and most preferably from about 35% to about 60% ethylene.
For other Component B copolymers, the comonomer contents will need to be adjusted depending on the specific properties desired. For example, for ethylene/hexene copolymers, Component B should contain at least 17% by weight hexene and at least 83% by weight ethylene.
Component B, preferably has a narrow molecular weight distribution Mw/Mn (xe2x80x9cMWDxe2x80x9d), i.e., lower than 5.0, preferably lower than 4.0, more preferably lower than 3.5, even more preferably lower than 3.0 and most preferably 2.5 or lower. These molecular weight distributions should be obtained in the absence of visbreaking or peroxide or other post reactor treatment designed to reduce molecular weight. Component B preferably has a weight average molecular weight (Mw as determined by GPC) of at least 100,000, preferably at least 150,000, and most preferably at least 200,000.
Component B preferably has an intrinsic viscosity greater than 1.00 dl/g, more preferably greater than 1.50 dl/g and most preferably greater than 2.00 dl/g. The term xe2x80x9cintrinsic viscosityxe2x80x9d or xe2x80x9cIVxe2x80x9d is used conventionally herein to mean the viscosity of a solution of polymer such as Component B in a given solvent at a given temperature, when the polymer composition is at infinite dilution. According to the ASTM standard test method D 1601-78, IV measurement involves a standard capillary viscosity measuring device, in which the viscosity of a series of concentrations of the polymer in the solvent at the given temperature are determined. For Component B, decalin is a suitable solvent and a typical temperature is 135xc2x0 C. From the values of the viscosity of solutions of varying concentrations, the xe2x80x9cvaluexe2x80x9d at infinite dilution can be determined by extrapolation.
Component B preferably has a composition distribution (CD) of greater than 60%, more preferably greater than 65%, even more preferably greater than 70%, even more preferably greater than 75%, still more preferably greater than 80%, and most preferably greater than 85%. CD defines the compositional variation among polymer chains in terms of ethylene (or other comonomer) content of the copolymer as a whole. The measurement of CD is described in detail U.S. Pat. No. 5,191,042 which is hereby fully incorporated by reference. CD is defined herein as the weight percent of the copolymer molecules having a comonomer content within 50% of the median total molar comonomer content.
As described in U.S. Pat. No. 5,191,042, CD is determined by first determining the mean ethylene (or other comonomer) content of the copolymer by a suitable test such as ASTM D-3900. Next, the copolymer sample is dissolved in solvent such as hexane and a number of fractions of differing composition are precipitated by the addition of incremental amounts of a liquid such as isopropanol in which the copolymer is insoluble. Generally from about 4 to 6 fractions are precipitated in this way and the weight and ethylene (or other comonomer) content of each fraction are determined after removing the solvent. From the weight of each fraction and its ethylene content, a plot is prepared of weight percent composition vs. cumulative weight percent of polymer, and a smooth curve is drawn through the points.
Component B of the ICPs preferably has low crystallinity, preferably less than 10% by weight of a crystalline portion, more preferably less than 5% by weight of a crystalline portion. Where there is a crystalline portion of Component B, its composition is preferably the same as or at least similar to (within 15% by weight) the remainder of Component B in terms of overall comonomer weight percent.
The ICPs of this invention are xe2x80x9creactor producedxe2x80x9d meaning Components A and B are not physically or mechanically blended together. Rather, they are interpolymerized in at least one reactor. The final ICP as obtained from the reactor or reactors, however, can be blended with various other components including other polymers.
The preferred melt flow rate (xe2x80x9cMFRxe2x80x9d) of these ICPs depends on the desired end use but is typically in the range of from about 0.2 dg/min to about 200 dg/min, more preferably from about 5 dg/min to about 100 dg/min. Significantly, high MFRs, i.e., higher than 50 dg/min are obtainable. MFR is determined by a conventional procedure such as ASTM-1238 Cond. L. The ICP preferably has a melting point of at least 145xc2x0 C., preferably at least 150xc2x0 C., more preferably at least 152xc2x0 C., and most preferably at least 155xc2x0 C.
The ICPs comprise from about 40% to about 95% by weight Component A and from about 5% to about 60% by weight Component B, preferably from about 50% to about 95% by weight Component A and from about 5% to about 50% Component B, even more preferably from about 60% to about 90% by weight Component A and from about 10 % to about 40% by weight Component B. In the most preferred embodiment, the ICP consists essentially of Components A and B. The overall comonomer (preferably ethylene) content of the total ICP is preferably in the range of from about 2% to about 30% by weight, preferably from about 5% to about 25% by weight, even more preferably from about 5% to about 20% by weight, still more preferably from about 5% to about 15% by weight comonomer.
A variety of additives may be incorporated into the ICP for various purposes. Such additives include, for example, stabilizers, antioxidants, fillers, colorants, nucleating agents and mold release agents.
The ICP compositions of this invention may be prepared by conventional polymerization processes such as a two-step process. It is conceivable, although currently impractical, to commercially produce ICPs in a single reactor. Each step may be independently carried out in either the gas or liquid slurry phase. For example the first step may be conducted in the gas phase and the second in liquid slurry or vice versa. Alternatively, each phase may be the same. Preferably the ICPs of this invention are produced in multiple reactors, preferably two or three, operated in series, Component B is preferably polymerized in a second, gas phase reactor. Component A is preferably polymerized first, in a liquid slurry or solution polymerization process.
In an alternative embodiment, Component A is made in at least two reactors in order to obtain fractions with varying melt flow rate. This has been found to improve the processability of the ICP.
As used herein xe2x80x9cstagexe2x80x9d is defined as that portion of a polymerization process during which one component of the ICP, Component A or Component B, is produced. One or multiple reactors may be used during each stage.
Hydrogen may be added to one or both reactors to control molecular weight, IV and MFR. The use of hydrogen for such purposes is well known to those skilled in the art.
Preferably a metallocene catalyst system is used to produce the ICP compositions of this invention. To date it appears that the most suitable metallocenes are those in the generic class of bridged, substituted bis(cyclopentadienyl) metallocenes, specifically bridged, substituted bis(indenyl) metallocenes known to produce high molecular weight, high melting, highly isotactic propylene polymers. Generally speaking, those of the generic class disclosed in U.S. Pat. No. 5,770,753 (fully incorporated herein by reference) should be suitable, however, it has been found that the exact polymer obtained is highly dependent on the metallocene""s specific substitution pattern.
We have found that the following racemic metallocenes are most suitable for preparing the ICP compositions of this invention: rac-dimethylsiladiyl(2-iPr,4-phenylindenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[1-naphthyl]indenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[3,5-dimethylphenyl]indenyl)2zirconium dichloride; rac-dimethylsiladiyl(2-iPr,4-[ortho-methyl-phenyl]indenyl)2zirconium dichloride; and rac-diphenylsiladiyl(2-methyl-4-[1-naphthyl]indenyl)2zirconium dichloride. It will be immediately apparent to those skilled in the art that certain modifications to these metallocene species are not likely to result in significantly modified ICP composition though activity or ease of synthesis may be impacted. While not wishing to be bound by theory, it is believed that the critical feature of these specific metallocenes is their substitution pattern on the base indenyl group. Thus, it is believed that changing the bridge, for example substituting carbon for silicon, or changing the metal to hafnium or titanium, or changing the metal dichloride to some other dihalide or dimethyl, will not significantly change the ICP compositions of this invention. On the other hand, substituting a group at any position on the indenyl for another or adding one or more groups or substitutents is likely to result in a significantly different composition which may or may not be an ICP of this invention.
Metallocenes are generally used in combination with some form of activator in order to create an active catalyst system. The term xe2x80x9cactivatorxe2x80x9d is defined herein to be any compound or component, or combination of compounds or components, capable of enhancing the ability of one or more metallocenes to polymerize olefins. Alkylalumoxanes such as methylalumoxane (MAO) are commonly used as metallocene activators. Generally alkylalumoxanes contain 5 to 40 of the repeating units:
R(AlRO )xc3x97AlR2 for linear species and
(AlRO )xc3x97for cyclic species
where R is a C1-C8 alkyl including mixed alkyls. Compounds in which R is methyl are particularly preferred. Alumoxane solutions, particularly methylalumoxane solutions, may be obtained from commercial vendors as solutions having various concentrations. There are a variety of methods for preparing alumoxane, non-limiting examples of which are described in U.S. Pat. Nos. 4,665,208, 4,952,540, 5,091,352, 5,206,199, 5,204,419, 4,874,734, 4,924,018, 4,908,463, 4,968,827, 5,308,815, 5,329,032, 5,248,801, 5,235,081, 5,103,031 and EP-A-0 561 476, EP-B1-0 279 586, EP-A-0 594-218 and WO 94/10180, each fully incorporated herein by reference.
Ionizing activators may also be used to activate metallocenes. These activators are neutral or ionic, or are compounds such as tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, which ionize the neutral metallocene compound. Such ionizing compounds may contain an active proton, or some other cation associated with, but not coordinated or only loosely coordinated to, the remaining ion of the ionizing compound. Combinations of activators may also be used, for example, alumoxane and ionizing activator combination, see for example, WO 94/07928.
Descriptions of ionic catalysts for coordination polymerization comprised of metallocene cations activated by non-coordinating anions appear in the early work in EP-A-0 277 003, EP-A-0 277 004 and U.S. Pat. No. 5,198,401 and WO-A-92/00333 (incorporated herein by reference). These teach desirable methods of preparation wherein metallocenes (bisCp and monoCp) are protonated by an anion precursor such that an alkyl/hydride group is abstracted from a transition metal to make it both cationic and charge-balanced by the non-coordinating anion. Suitable ionic salts include tetrakis-substituted borate or aluminum salts having fluorided aryl-constituents such as phenyl, biphenyl and napthyl.
The term xe2x80x9cnoncoordinating anionxe2x80x9d (NCA) means an anion which either does not coordinate to said cation or which is only weakly coordinated to said cation thereby remaining sufficiently labile to be displaced by a neutral Lewis base. xe2x80x9cCompatiblexe2x80x9d noncoordinating anions are those which are not degraded to neutrality when the initially formed complex decomposes. Further, the anion will not transfer an anionic substituent or fragment to the cation so as to cause it to form a neutral four coordinate metallocene compound and a neutral by-product from the anion. Particularly useful noncoordinating anions are those which are compatible, stabilize the metallocene cation in the sense of balancing its ionic charge in a +1 state, yet retain sufficient lability to permit displacement by an ethylenically or acetylenically unsaturated monomer during polymerization.
The use of ionizing ionic compounds not containing an active proton but capable of producing both the active metallocene cation and a noncoordinating anion is also known. See, for example, EP-A-0 426 637 and EP-A-0 573 403 (incorporated herein by reference). An additional method of making the ionic catalysts uses ionizing anion precursors which are initially neutral Lewis acids but form the cation and anion upon ionizing reaction with the metallocene compounds, for example the use of tris(pentafluorophenyl)borane. See EP-A-0 520 732 (incorporated herein by reference). Ionic catalysts for addition polymerization can also be prepared by oxidation of the metal centers of transition metal compounds by anion precursors containing metallic oxidizing groups along with the anion groups, see EP-A-0 495 375 (incorporated herein by reference).
Where the metal ligands include halogen moieties (for example, bis-cyclopentadienyl zirconium dichloride) which are not capable of ionizing abstraction under standard conditions, they can be converted via known alkylation reactions with organometallic compounds such as lithium or aluminum hydrides or alkyls, alkylalumoxanes, Grignard reagents, etc. See EP-A-0 500 944 and EP-A1-0 570 982 (incorporated herein by reference) for in situ processes describing the reaction of alkyl aluminum compounds with dihalo-substituted metallocene compounds prior to or with the addition of activating anionic compounds.
Methods for supporting ionic catalysts comprising metallocene cations and NCA are described in U.S. Pat. No. 5,643,847, U.S. patent application Ser. No. 09/184,358, filed Nov. 2, 1998 and U.S. patent application Ser. No. 09/184,389, filed Nov. 2, 1998 (all fully incorporated herein by reference).
When the activator for the metallocene supported catalyst composition is a NCA, preferably the NCA is first added to the support composition followed by the addition of the metallocene catalyst. When the activator is MAO, preferably the MAO and metallocene catalyst are dissolved together in solution. The support is then contacted with the MAO/metallocene catalyst solution. Other methods and order of addition will be apparent to those skilled in the art.
The catalyst systems used to prepare the compositions of this invention are preferably supported using a porous particulate material, such as for example, talc, inorganic oxides, inorganic chlorides and resinous materials such as polyolefin or polymeric compounds.
Preferably, the support materials are porous inorganic oxide materials, which include those from the Periodic Table of Elements of Groups 2, 3, 4, 5, 13 or 14 metal oxides. Silica, alumina, silica-alumina, and mixtures thereof are particularly preferable. Other inorganic oxides that may be employed either alone or in combination with the silica, alumina or silica-alumina are magnesia, titania, zirconia, and the like.
Preferably the support material is porous silica which has a surface area in the range of from 10 to 700 m2/g, a total pore volume in the range of from 0.1 to 4.0 cc/g and an average particle size in the range of from 10 to 500 xcexcm. More preferably, the surface area is in the range of from 50 to 500 m2/g, the pore volume is in the range of from 0.5 to 3.5 cc/g and the average particle size is in the range of from 20 to 200 xcexcm. Most desirably the surface area is in the range of from 100 to 400 m2/g, the pore volume is in the range of from 0.8 to 3.0 cc/g and the average particle size is in the range of from 30 to 100 xcexcm. The average pore size of typical porous support materials is in the range of from 10 to 1000 xc3x85. Preferably, a support material is used that has an average pore diameter of from 50 to 500 xc3x85, and most desirably from 75 to 350 xc3x85. It may be particularly desirable to dehydrate the silica at a temperature of from 100xc2x0 C. to 800xc2x0 C. anywhere from 3 to 24 hours.
The metallocenes, activator and support material may be combined in any number of ways. Suitable support techniques are described in U.S. Pat. Nos. 4,808,561 and 4,701,432 (each fully incorporated herein by reference). Preferably the metallocenes and activator are combined and their reaction product supported on the porous support material as described in U.S. Pat. No. 5,240,894 and WO 94/28034, WO 96/00243, and WO 96/00245 (each fully incorporated herein by reference). Alternatively, the metallocenes may be preactivated separately and then combined with the support material either separately or together. If the metallocenes are separately supported, then preferably, they are dried then combined as a powder before use in polymerization.
Regardless of whether the metallocenes and their activator are separately precontacted or whether the metallocenes and activator are combined at once, the total volume of reaction solution applied to porous support is desirably less than 4 times the total pore volume of the porous support, more desirably less than 3 times the total pore volume of the porous support and even more desirably in the range of from more than 1 to less than 2.5 times the total pore volume of the porous support. Procedures for measuring the total pore volume of porous support are well known in the art. One such method is described in Volume 1, Experimental Methods in Catalyst Research, Academic Press, 1968, pages 67-96.
Methods of supporting ionic catalysts comprising metallocene cations and noncoordinating anions are described in WO 91/09882, WO 94/03506, WO 96/04319 and in co-pending U.S. Ser. No. 08/248,284, filed Aug. 3, 1994 (incorporated herein by reference). The methods generally comprise either physical adsorption on traditional polymeric or inorganic supports that have been largely dehydrated and dehydroxylated, or using neutral anion precursors that are sufficiently strong Lewis acids to activate retained hydroxy groups in silica containing inorganic oxide supports such that the Lewis acid becomes covalently bound and the hydrogen of the hydroxy group is available to protonate the metallocene compounds.
The supported catalyst system may be used directly in polymerization or the catalyst system may be prepolymerized using methods well known in the art. For details regarding prepolymerization, see U.S. Pat. Nos. 4,923,833 and 4,921,825, EP 0 279 863 and EP 0 354 893 each of which is fully incorporated herein by reference.
While the present invention has been described and illustrated by reference to particular embodiments, it will be appreciated by those of ordinary skill in the art, that the invention lends itself to many different variations not illustrated herein. For these reasons, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.
Although the appendant claims have single appendencies in accordance with U.S. patent practice, each of the features in any of the appendant claims can be combined with each of the features of other appendant claims or the main claim.